The Origin of Humankind Read Online Free PDF

and—presumably because they do not have reason to fight with one another—the males have small canine teeth. The small canines in the earliest humans may be an indication that, like gibbons, they formed male-female pairs, Lovejoy argued. The social and economic bonds of the provisioning arrangement would in turn have driven an increase in the size of the brain.
    Lovejoy’s hypothesis, which enjoyed considerable attention and support, is powerful because it appeals to fundamental biological issues, not cultural ones. It has weak points, however; for one thing, monogamy is not a common social arrangement among technologically primitive people. (Only 20 percent of such societies are monogamous.) The hypothesis was therefore criticized for seeming to draw on a trait of Western society, not one of hunter-gatherers. A second criticism, perhaps more serious, is that the males of the known early human species were about twice the size of females. In all species of primate that have been studied, this great difference in body size, known as dimorphism, correlates with polygyny, or competition among the males for access to females; dimorphism is not seen in monogamous species. For me, this fact alone is sufficient to sink a promising theoretical approach, and an explanation other than monogamy must be sought for the small canines. One possibility is that the mechanism of masticating food required a grinding rather than a slicing motion; large canines would impair such a motion. Lovejoy’s hypothesis enjoys less support now than it did a decade ago.
    The second major bipedalism theory is much more persuasive, partly for its simplicity. Proposed by the anthropologists Peter Rodman and Henry McHenry, of the University of California, Davis, the hypothesis states that bipedalism was advantageous in the changing environmental conditions because it offered a more efficient means of locomotion. As the forests dwindled, food resources in woodland habitats, such as fruit trees, would have become too dispersed to be efficiently exploitable by conventional apes. According to this hypothesis, the first bipedal apes were human only in their mode of locomotion. Their hands, jaws, and teeth would have remained apelike, because their diet had not changed, only their manner of procuring it.
    To many biologists, this proposal initially seemed unlikely; researchers at Harvard University had shown some years earlier that walking on two legs is less efficient than walking on four. (This shouldn’t be a surprise to anyone with a dog or a cat; both run, embarrassingly, much faster than their owners.) The Harvard researchers had, however, compared the energy efficiency of bipedalism in humans with that of quadrupedalism in horses and dogs. Rodman and McHenry pointed out that the proper comparison should have been between humans and chimpanzees. When these comparisons are done, it turns out that bipedalism in humans is more efficient than quadrupedalism in chimpanzees. An energy-efficiency argument as a force of natural selection in favor of bipedalism, they concluded, is therefore plausible.
    There have been many other suggestions for the factors that drove the evolution of bipedalism, such as the need to look over tall grass while monitoring predators and the need to adopt a more efficient posture for cooling during daytime foraging. Of them all, I find Rodman and McHenry’s the most cogent, because it is firmly biologically based and fits the ecological changes that were occurring when the first human species evolved. If the hypothesis is correct, it will mean that when we find fossils of the first human species, we may fail to recognize them as such, depending on which bones we have. If the bones are those of the pelvis or lower limbs, then the bipedal mode of locomotion will be evident, and we will be able to say “human.” But if we were to find certain parts of the skull, jaw, or some teeth, they might look just like those of an ape. How would we